Apparatus for controlling and modulating engine functions

ABSTRACT

Changes in barometric pressure and/or temperature can adversely affect a number of engine functions. Apparatus is disclosed for controlling and modulating a number of engine functions, including carburetor idle fuel and main fuel supply, fuel enrichment during acceleration and wide open throttle operation, as well as vacuum applied to such vacuum motors associated with the engine as the carburetor choke, the spark advance and automatic transmission shift. Calibration features are provided for establishing a base condition of operation as well as individual calibration of all the various functions listed together with the ability to make base calibration changes by application of an external signal.

United States Patent 191 Brown et al.

[ Dec. 31, 1974 APPARATUS FOR CONTROLLING AND MODULATING ENGINE FUNCTIONS i [75] Inventors: Morris C. Brown, Florissant; Forrest W. Cook, both of Webster Groves,

Mo.; Ralph E. Kalert, Granite City, 111.; Arthur C. Vollmer, St. Charles; Jerry II. Winkley, St. Louis, both of [73] Assignee: ACF Industries, Incorporated, New

York, NY. 22 Filed: Feb. 9, 1973 [21] Appl. No.: 331,262

[52] US. Cl. 261/39 A, 236/93, 261/121 B [51] Int. Cl. F02m 1/14 [58] Field of Search 261/39 A, 12l B; 267/177,

[56] References Cited UNITED STATES PATENTS l/1941 Kratzer 261/39 A 2,611,595 9/1952 261/39 A 2,621,910 12/1952 V012 et al. 261/39 A 2,631,024 3/1953 Beardsley, .lr. 261/39 A 3,127,147 3/1964 Spangenbcrg 261/177 3,667,740 6/1972 Mortstedt 261/39 A 3,764,120 10/1973 lmai 261/121 B Primary Examiner-Tim R. Miles [57] ABSTRACT of operation as well as individual calibration of all the various functions listed together with the ability to make base calibration changes by application of an external signal.

4 Claims, 19 Drawing Figures PATENIEDUEB31 1914 31.857, 908 I SHEET 2 BF 5 v 2 /6 K 4 a2 a0 a4 FIGURE 5. 7 a: 4/75 /a FIGURE 6.

FIGURE 7.

PATENTED M831 1974 SHEET 30F 5 FIGURE 8.

FIGURE 9.

PATENIED 3, 857, 908 saw u 0F 5 FIGURE II.

FIGURE I3.

FIGURE I9. 7

PATENTEDUECSIIBH SHEET 5 BF 5 FIGURE I6.

FlGU RE l5.

FIGURE ls.

FIGURE l7.

APPARATUS FOR CONTROLLING AND MODULATING ENGINE FUNCTIONS BACKGROUND OF THE INVENTION There are a number of functions of the internal combustion engine that are affected, usually adversely, by changes in operating conditions such as barometric pressure, ambient temperature and frequently one or more temperatures associated with the engine itself. In the case of stationary engines, and other engines operating under essentially constant load conditions, adjustments can be made to achieve efficient operation of the engine under these more or less constant conditions. In the case of the automotive engine, no single set of adjustments are capable of compensating for the constantly changing variables surrounding the engine.

Among the several engine functions that can be affected by changes in altitude and temperature, the idle fuel and main fuel systems of the conventional carburetor are very important. These fuel metering systems are severely affected by changes in altitude; so much so, that with an unmodified carburetor the engine will receive an unnecessarily rich mixture whenever there is any great departure from sea level. Customarily, this situation has been accepted and/or tolerated with the exception that if an automotive vehicle is normally used at higher altitudes a permanent change is made to reduce the size of the metering jet or to install a larger metering rod, or both, and such modification allows the engine to operate correctly so long as the vehicle is kept at substantially the same altitude. If this vehicle is driven to sea level conditions, then the mixture may be unduly lean. In a somewhat similar manner, changes in temperature which directly affect the temperature of the fuel as well as that of the air entering the carburetor will also cause undesirable changes in the air/fuel ratio discharged by the carburetor to the engine. For automotive engines, little has been done to alleviate these conditions other than the use of a hot idle compensator which can admit some additional air to the intake manifold when ambient conditions have exceeded some preselected temperature and this addition of air without additional fuel has the affect of leaning the mixture out at the higher ambient temperature. While automotive carburetors normally have made no compensation for the above-mentioned variables, many aircraft carburetors over the years have necessarily embodied altitude compensation, but because of the considerable differences in the carburetors used on aircraft with respect to those used on automobiles, the overall approach to correcting fuel ratio is not well suited to that of the automotive carburetor.

Power enrichment is another important carburetor function that is adversely affected by changes in altitude and for which there has been no compensation provided in commercial carburetors. Power enrichment customarily is provided by opening an auxiliary fuel passage or by moving a metering element so that additional fuel can enter the main fuel system. In both cases, the manifold vacuum is applied to one side of a diaphragm or piston and when manifold vacuum is high only normal fuel quantities are allowed to enter the main fuel system. When manifold vacuum becomes low then the movable member shifts into another position, thus allowing enrichment fuel to pass into the fuel system. The movable member (diaphragm or piston) is normally biased by a spring to move toward the enrichment position and the bias of this spring is overcome by the manifold vacuum. If the spring is so adjusted that it will cause enrichment whenever manifold vacuum drops below about 6 inches mercury, then no enrichment occurs until the engine is heavily loaded and usually a wide-open throttle condition has been reached. On the other hand, when the same vehicle is operated at altitudes of 5,000 feet or more, the power enrich ment may come into play even though the engine is relatively lightly loaded. This condition is caused by the fact that barometric pressure at altitudes is considerably less than that at sea level and this reduction in ambient pressure is reflected in the intake manifold as a lesser degree of manifold vacuum for normal operation. Accordingly, as the engine is increased in its normal loading, the pressure condition necessary for power enrichment may be reached far in advance of any actual need for the enrichment.

There are other engine and vehicle functions that are also dependent upon manifold vacuum and which functions can be adversely affected by changes in altitude. One such engine function is the advance and retard of the spark ignition which is normally done by a diaphragm motor. Again, a considerable change in altitude may cause the diaphragm motor to shift the spark advance at a time when the engine does not require such a shift and this, then becomes an undesirable result. In a similar fashion, some automatic transmissions are equipped with a diaphragm actuated motor for assisting in the shifting of the transmission mechanism. As is the case of the spark advance, the automatic transmission vacuum motor may come into play or fall out of play at an undesirable operating condition.

BRIEF DESCRIPTION OF THE INVENTION In order to make the compensation for the various engine and automative functions described above, there is provided a compensation unit which can be installed in the vehicle or made integral with the carburetor as desired and this unit meters air into the various systems involved in such a manner as to automatically compensate for changes in engine operating characteristics caused by changes in atmospheric pressure and- /or temperature.

Basically, the compensation unit includes a capsule which is temperature and/or barometric sensitive to change its dimensions as the altitude or the temperature varies. This capsule moves a plate to which are attached a number of metering elements. One group of metering elements are arranged in such a manner as to increase air flow as the altitude increases and also increase air flow as temperature increases. Another metering element is arranged to operate in the opposite direction so that maximum air flow occurs at low altitude and/or temperature and a decreasing quantity of air flows as the altitude and/or temperature increases. The first-mentioned group of metering elements provide an air bleed into one or more of the fuel systems of the carburetor to overcome the tendency toward richness as temperature and altitude increase. Another metering element is adapted to admit larger quantities of air at sea level and standard temperatures with a reduced quantity of air as the altitude and/or temperature increases. This least mentioned metering element serves to reduce what would normally be a high manifold vacuum applied to some operative function of the engine at low altitudes and low temperatures and to reduce the air admitted with changes in those variables so that the vacuum actually applied will remain substantially constant, irrespective of any change in altitude or temperature.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1 there is shown an altitude and/or temperature compensating unit 10 attached to a carburetor 12 by a plurality of conduits 14, 16, 18 and 20, which illustrate one realization of the invention. Carburetor 12 is suitably mounted on an intake manifold 13 which delivers the usual air/fuel mixture to an engine 15. Products of combustion are discharged from the engine into an exhaust pipe 17 and thence to a muffler 19 from which the products are discharged into the atmosphere. An air cleaner or filter 21 is installed on carburetor 12 so that only clean filtered air is delivered to the intake of the internal combustion engine.

Referring now to FIGS. 2, 3 and 4, compensating unit 10 is shown in greater detail. The unit comprises a body portion 22 having a cover 23 which cover seals the unit from the atmosphere. Conduit 14, which in this embodiment may be a rubber hose, is attached to a nipple 24 for conducting clean air to the interior of unit 10. Thus, the interior of unit 10 has air at substantially atmospheric pressure inside it at all times. Any deviation from absolute atmospheric pressure will be due to any pressure drop that may occur across the filter element and air cleaner 21. Within housing 22, there is an ambient air condition responsive device 30 which, in this instance, is a capsule of the bellows type which is conditioned to have within it a predetermined air pressure which, for barometric or altitude compensation represents a high degree of evacuation of the interior of the capsule. Additionally, there may be provided on the interior of capsule 30 a spring 32 which, through proper biasing, makes the capsule responsive to give the desired movement for a given change. High evacuation makes the capsule almost totally responsive to barometric change, while partial evacuation causes the capsule to respond to both barometric and temperature change. Accordingly, the spring bias and degree of evacuation will be chosen to achieve predetermined requirements. The base of capsule 30 is secured to the housing 22 in any convenient manner, in this instance by way of a boss 33. A detent 34 is provided at the upper end of capsule 30 for receiving an adjustment screw 36 as will be hereinafter described. A plate 40 is installed in an upper portion of housing 22 and is pivoted at 42 in such a manner that it positions adjustment screw 36 above unit 30 so that changes of length in unit 30 occasioned by changes in barometric pressure and- /or temperature will cause plate 40 to move upwardly and downwardly about the pivot 42.

A plurality of adjustment screws other than screw 36 are carried by plate 40. These are identified as 44, 46, 48 and 50. Each such adjustment screw is provided for the purpose of making final adjustment and calibration of the metering elements and biasing arrangements used in the device. Screw 44 at its lower extremity contacts a metering pin 52 which, by way of conduit 16, controls the quantity of air bled into the main fuel system. Such bleed air, when increased in quantity serves to diminish the amount of fuel delivered to the carburetor by the main fuel nozzle. When the bleed air is reduced or cut off, a greater quantity of fuel is made available through the main nozzle. Metering pin 52 is urged in an upward direction by a biasing spring 54. Clean air from the interior of housing 22 is admitted into the vicinity of metering pin 52 by an air passageway 53 so that as pin 52 rises. the tapered end portion uncovers the air entry passageway to admit air into conduit 16 and thence, by way of conduit 16 to a passageway 55 in the carburetor itself, which communicates with a portion of the main fuel system and, in this instance, the anti-perk well 56. Air passing into antiperk well 56 then mixes with the fuel in passageway 57 so that a mixture of air and fuel are discharged by nozzle 58 into the boost venturi 60 of the carburetor. The fuel, of course, is supplied from a constant level fuel bowl 62 and enters the main fuel passage by way of a metering orifice 64 which is controlled in part by a metering rod 66.

Air is bled into the idle fuel system in a manner substantially the same as that described above with respect to the main fuel system. Referring to FIG. 6, it will be seen that idle fuel is drawn by a passage from the main fuel passage 57 of FIG. 5 and passes upwardly through a dip tube 71 and thence. through a restriction 72 where it is mixed with a quantity of air supplied by way of orifice 73, the resultant mixture then passes through restriction 74 where additional air can be added by way of restriction 75, thence downwardy by way of passage 77 to a cross-passage 78 which terminates in the bore of the carburetor adjacent the throttle valve.

Communication with the compensating unit is established by way of restriction 79 and conduit 18 which connects with the compensating unit and is supplied with metered air by way of metering pin 80 which is biased by way of a spring 81 in an outwardly direction and which. as previously mentioned, is adjusted by way of adjustment screw 46 carried on plate 40. It is to be mentioned that both metering pins 52 and 80 are provided with a taper such that as plate 40 moves upwardly, greater quantities of air are admitted into conduits 16 and 18 thereby reducing the quantity of fuel discharged by the main and idle fuel systems.

A final engine function that is accomplished by the compensating unit of the invention is illustrated by two different embodiments in FIG. 7. In that Figure there are shown two vacuum responsive devices in the form of diaphragm units. One of these controls the step-up by which the carburetor is caused to provide enrichment fuel under heavy-load or wide-open throttle conditions and the other is a diaphragm motor such as can be used to control spark advance or, in some instances, the shift mechanism of an automatic transmission. It is to be recognized that any or all of these vacuum responsive units could comprise a piston instead of a diaphragm mechanism.

As showin in FIG. 7, conduit 20 is attached to the carburetor by means of a tube 85 which intersects a passage 86 in the throttle flange ofthe carburetor. Passage 86 enters the bore of the throttle of the carburetor below the throttle valve 88. By means of a branch passage, conduit 86 communicates with a space 90 below the diaphragm of a diaphragm-type accelerating pump. The diaphragm is shown at 91 and is attached by suitable retaining means to a stem 92 which is biased in an upwardly direction by spring 93. A cavity 94 on the upper side of the diaphragm is utilized for the acceleration fuel provided by the diaphragm pump. In this in stance, the inlet and outlet of fuel to the space 94 is not shown for convenience sake. Stem 92 is provided with a carrier bar 96 which supports the metering rod 66. In the normal operation of the pump, manifold vacuum acting in space 90 pulls the diaphgragm down and this permits the pumping space 94 to fill with fuel while at the same time drawing the metering rod 66 down into the meteringjet 64 to reduce the annular clearance between the taper of the metering rod and the interior of the jet. Upon a reduction of vacuum, as would come about if throttle 88 were opened more widely, spring 93 overcomes the vacuum and causes the diaphragm to move upwardly so that fuel is discharged into the carburetor from the accelerating pump and at the same time, metering rod 66 is drawn up to provide a greater annular clearance, thus providing additional fuel to the carburetor for enrichment purposes. This is commonly referred to as step-up.

Referring now to FIG. 4, it will be seen that conduit 20 communicates with air metering means in the form of a metering rod 100 which is provided at its lowermost extremity with a reverse taper metering section. As with the metering rods described earlier, rod 100 moves up and down under the influence of biasing spring 102 and carrier plate 40. Adjustment screw 48 makes final calibration adjustments of the relationship of metering rod 100 with its metering orifice. Air is provided from the interior of compensating unit to the metering zone by way of a passage 104.

Returning to FIG. 7, there is shown an additional engine control function connected by a pipe 110 to passage 86. Pipe 110 communicates with a vacuum motor 112 which is shown as a diaphragm unit but which might be a piston-type device. Diaphragm unit 112 is provided with a diaphragm 114 biased in a direction away from the source of vacuum by a spring 116. An operating arm 118 is attached to the opposite side from the biasing spring to the diaphragm. Arm 118 can be connected to any selected function, as for example, the spark advance of the ignition distributor or to the shift mechanism of an automatic transmission. Each of these engine functions may be required to alter operating characteristics as the vehicle is driven into changing conditions of altitude or temperature. Thus, the vacuum motor 112 which receives a controlled vacuum signal which is dependent upon the altitude and/or temperature can be made to function in a desired and programmed manner. Although motor 112 is shown connected to the same system as the accelerator pump and step-up of the carburetor, it could be divorced from that system as would be obvious merely through the provision of a separate air metering element corresponding to element 100 and a separate source of vacuum together with a suitable connecting passage corresponding to passage 20.

The foregoing description has described a system primarily for use in a single barrel carburetor in which the altitude/temperature compensating unit is separated from the carburetor and engine and connected to the necessary components by means of flexible tubes or hoses. It will be apparent that to the extent the altitude/temperature unit controls carburetor functions, the unit can be integrated with the carburetor structure and the various hoses replaced with integral passages in the carburetor body structure. In such an embodiment, the only remote locations connected by tubes or hoses would be to the other engine functions such as distributor vacuum motors and/or automatic transmission control motors.

In the remaining figures of drawing, the basic concepts of the invention are applied to a four-barrel carburetor and, for the most part, these same concepts would be true of a twobarrel carburetor. In making the description of the four-barrel carburetor, numbers similar to that used in the single barrel carburetor will be used, excepting that all numbers will be three-digit numbers in place of mostly two-digit numbers used earlier, as for example, the number 14 will become 214, 20 will become 220, etc. A further difference between the four-barrel embodiment and the single barrel embodiment is that the passageways which were formerly separate conduits, such as flexible rubber tubing, now are passages formed or drilled integrally in the carburetor body and the altitude/temperature compensation unit is also mounted directly on the carburetor body.

Referring now to FIG. 8, there is shown the base of a compensating unit 220 mounted on the body of a four-barrel carburetor 212. A cover 223 is provided for covering up the operative mechanism of the compensator unit. The four-barrel carburetor 212 has two primary barrels and two secondary barrels. The primary barrels are each equipped with an idle fuel system and a main fuel system while the secondary barrels have only a single main fuel system each. Since each of the fuel systems can be managed by single metering pin, the unit 210 duplicates the metering capability of the compensating unit 10 of FIG. 1 with the addition ofone additional capability for the secondary fuel nozzles. A passage 214 communicates at one end with the air horn of the carburetor and, thus, with the space inside the air cleaner and its other end with the interior of compensating unit 210. In this fashion, clean air is supplied for all air metering functions. This last is shown also in FIG. 10.

Also shown in FIG. 8 is a passageway 216 which extends from the interior of unit 210 to a branch extending to each of the two main nozzle wells indicated at 256. Similarly, a passage 218 is branched to extend to the two idle fuel passages 277. A passage 213 extends to a branching point where it separates and branches out to secondary fuel wells 215. It is to be remembered that all of the fuel passages and fuel systems in the fourbarrel version are substantially similar to the single barrel version shown in FIGS. 1 through 7. Principal difference being the secondary fuel nozzle and secondary fuel well which are at least similar to the primary fuel nozzle and well.

Multiple barrel carburetors are normally provided with a pair of metering rods 266 which cooperate with a metering jet similar to jet 64 of FIG. 5. In this embodiment shown, the metering rod 266 is controlled in part by a vacuum actuated piston 291 which receives vacuum by way of passage 286. The passage 2 86 communicates with the intake manifold at a point beneath the throttle valve and communicates also with a passage 220 which terminates in the compensating unit 210 where an air metering pin controls the quantity of air that is admitted into the vacuum passage 286 to regulate the vacuum therein.

Referring now to FIG. 13, vacuum piston 291 can also be forced into an upward position by means of a rod 300 which is driven by a leaf 302 which in turn is driven by a cam 304 on throttle shaft 306. Thus, when throttle shaft 6 is rotated in a direction to open the throttle, the cam 304 will move leaf 302 upwardly to in turn move rod 300 which in turn will lift the metering pin 266 the desired amount. In this manner, additional fuel can be delivered to the main nozzles of the primary barrel as the throttles are opened. On the other hand, if there is a sudden loss of manifold vacuum, as sometimes occurs, piston 291 is allowed to rise upwardly and this in turn lifts the metering rod 266 to provide the fuel enrichment usually desired under these conditions.

Referring now to FIGS. 14 through 18, the compensating unit 210 is quite similar to the unit 10 of FIGS. 2, 3 and 4, and contains all of the same features, although housed somewhat differently. Air conduits 213, 214, 216, 218 and 220 are connected to a carburetor as shown in FIG. 8. Plate 240 is hinged at 242 and carries a plurality of adjustment screws. Adjustment screw 236 makes any needed corrections or adjustments to the contact with barometric and/or temperature capsule 230. Adjustment screws 244, 246 and 248 are provided to position metering pins in the same fashion as screws 44, 46 and 48' of FIGS. 3 annd 4. Additionally, biasing adjustment screw 250 adjusts tension on a spring in the same manner as does screw 50 of FIG. 3. A further metering screw 245 is provided to adjust the position of the metering pin which controls the air delivered to the secondary fuel nozzles by way of conduit 13 and, as explained earlier, this is in the same manner as the air delivered to the primary main nozzles.

In FIGS. 8 through 19, the 200 series of numbers corresponds as nearly as possible with the below 100 series of numbers in FIGS. 1 through 7. It is believed unnecessary to give a detailed description of the various features of FIG. 8 through 19 and a brief description is believed sufficient.

FIG. 14 corresponds roughly to FIG. 2, with the exception that it is configured for a multibarrel carburetor. FIG. 15 corresponds to FIG. 3. FIG. 16 illustrates a means by which spring 251 can be adjusted by means of screw 250 and also by means of a screw 253 which adjusts the bottom end of the spring 251. In a similar manner, spring 251 can be adjusted by an external means, which, for example, could be a temperature sensor 255 in communication with an exhaust manifold 257 which by means of a thermal motor 259, can adjust an arm 261 which in turn will adjust the spring 251. Thus, as exhaust temperature changes, the bias on spring 251 will be adjusted in accordance with engine operation to put a bias on plate 240 to thereby change the quantity of air bled into the fuel metering systems in accordance with engine demands. FIG. 11 is similar to FIG. 5, excepting that the fuel nozzle 271 is a secondary main fuel nozzle and in all other respects, the air bleed into the nozzle is similar to that of FIG. 5. FIG. 13 is similar to the righthand portion of FIG. 7 in that it shows a means for raising or lowering a metering rod 266 in accordance with the position of the throttle. 306 is the primary throttle shaft and 304 is a cam on that shaft. The cam in turn moves a lever 302 which raises and/or lowers a rod 300 which is in direct connection with metering rod 266.

FIGS. 17 and 18 illustrate metering pins moved by plate 240. In FIG. 17 metering pin 320 functions in the same manner as metering pin 100 of FIG. 4 to reduce the quantity of air flowing through a passage 322 which communicates with a motor such as diaphragm motor 112 of FIG. 7. In a similar manner, metering rod 280 of FIG. 18 functions much the same as metering rod of FIG. 4 to increase the amount of air bled into the fuel system when plate 240 rises under the influence of reduced barometric pressure or increased temperature and rod 280 can be adjusted as desired by screw 250 and its associated nut.

From the foregoing it can be seen that a capsule sensitive to barometric pressure and/or temperature can be utilized to adjust air bleeds into various operative functions of an engine to thereby control said functions as a result of changes in atmospheric pressure and/or temperature. In general, it can be said that whether the engine be equipped with a one-barrel, two-barrel, or four-barrel carburetor, the air bleeds can be compensated in such a manner that, with increasing altitude or decreasing barometric pressure, the fuel supplied to the carburetor will be reduced in accordance with such change and other operative functions of the vehicle such as transmission shifts and/or spark advance or retard can be accomodated in such a manner as to achieve optimum operation of the engine and vehicle.

I claim: 1. In an internal combustion engine having a plurality of engine functions operable by the suction created by the natural aspiration of said engine during running condition, the improvement comprising;

an attachment connected to said engine communicating on one side with ambient air and on another side with at least one suction controlled passage leading to at least one engine function controlling means. said attachment including a bellows responsive to barometric and/or temperature changes of said atmospheric air to change a dimension of said bellows, 7

said bellows being mounted in a housing, said housing also including a pivotable plate surmounting said bellows,

at least one metering means being connected to said plate, said metering means being adapted to meter ambient air to said at least one engine function controlling means and said pivotable plate being pivoted at one end thereof,

said plate adjustably contacting said bellows at an intermediate position along the length of said plate, said plate also having adjustment biasing means at the end opposite said pivoted end.

2. Apparatus according to claim 1 in which said biasing means comprises a spring attached at one end thereof to said housing and at the other end thereof to an adjustment screw.

3. Apparatus according to claim 2 in which said adjustment screw is threaded through an opening in said plate whereby to adjust the bias of said biasing spring.

4. Apparatus according to claim 3 in which said adjustment screw includes an enlarged head thereon and said spring comprises a series of coils, said enlarged head constituting means for engaging different coils of said biasing spring whereby through selection of a correct coil engagement, the rate of said spring can be selected in accordance with a predetermined set of ambient conditions. 

1. In an internal combustion engine having a plurality of engine functions operable by the suction created by the natural aspiration of said engine during running condition, the improvement comprising; an attachment connected to said engine communicating on one side with ambient air and on another side with at least one suction controlled passage leading to at least one engine function controlling means. said attachment including a bellows responsive to barometric and/or temperature changes of said atmospheric air to change a dimension of said bellows, said bellows being mounted in a housing, said housing also including a pivotable plate surmounting said bellows, at least one metering means being connected to said plate, said metering means being adapted to meter ambient air to said at least one engine function controlling means and said pivotable plaTe being pivoted at one end thereof, said plate adjustably contacting said bellows at an intermediate position along the length of said plate, said plate also having adjustment biasing means at the end opposite said pivoted end.
 2. Apparatus according to claim 1 in which said biasing means comprises a spring attached at one end thereof to said housing and at the other end thereof to an adjustment screw.
 3. Apparatus according to claim 2 in which said adjustment screw is threaded through an opening in said plate whereby to adjust the bias of said biasing spring.
 4. Apparatus according to claim 3 in which said adjustment screw includes an enlarged head thereon and said spring comprises a series of coils, said enlarged head constituting means for engaging different coils of said biasing spring whereby through selection of a correct coil engagement, the rate of said spring can be selected in accordance with a predetermined set of ambient conditions. 